It is well known that isobutylene may be reacted with methanol over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) and isoamylenes may be reacted with methanol over an acidic catalyst to produce tertiary amyl methyl ether (TAME). The reaction is a useful preparation for these valuable gasoline octane enhancers and is typical of the reaction of the addition of lower alkanol to the more reactive tertiary alkenes, or iso-olefins, of the type R.sub.2 C.dbd.CH.sub.2 or R.sub.2 C.dbd.CHR under mild conditions to form the corresponding tertiary alkyl ethers. The feedstock for the etherification reaction may be taken from a variety of refinery process streams such as the unsaturated gas plant of a fluidized bed catalytic cracking operation containing mixed light olefins, preferably rich in isobutylene and isopentenes or isoamylene.
Generally, it is known that asymmetrical C.sub.5 -C.sub.7 alkyl tertiary alkyl ethers are particularly useful as octane improvers for liquid fuels, especially gasoline. MTBE, ethyl t-butyl ether (ETBE), isopropyl t-butyl ether (IPTBE) and TAME are known to be high octane ethers The article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The blending octane numbers of MTBE when added in an amount of 10% to a typical unleaded gasoline are: RON is 118, MON is 101, R+M/2 is 109. Increasing demand for high octane gasolines blended with high octane ethers as octane boosters and supplementary fuels has created a significant demand for these ethers, especially MTBE and TAME.
The production of alkyl tertiary alkyl ethers is known to be catalyzed by Lewis acids such as AlCl.sub.3 and BF.sub.3, mineral acids and strong organic acids. The reaction equilibrium favors the formation of ether at low reaction temperature, but reaction kinetics at low temperature limits the degree of conversion. In practice, this limitation can be overcome by carrying out the reaction using an excess of either the alkanol or iso-olefin reactant. When excess alkanol is used conversions of iso-olefin above ninety percent are realized. But using excess alkanol results in difficult and costly operations to remove unreacted alkanol from the reaction products. On the other hand, employing an excess of olefin favors the formation of undesirable reaction by-products, such as olefin dimer.
In U.S. Pat. No. 4,605,787 to Chu, incorporated herein by reference, a process is disclosed for the preparation of alkyl tert-alkyl ethers using acidic zeolite catalyst, including zeolite Beta. It has been found that zeolite catalyst suppresses the formation of diisobutylene by-product in the production of MTBE from isobutylene.
In EP 0,055,045 to Daniels a process is disclosed for the production of MTBE using zeolite Beta catalyst. The process employs a large excess of methanol and achieves a conversion of isobutene of about 87 mole % with high selectivity to MTBE.
It is well known that zeolite catalysts, including zeolite Beta, are highly effective under a broad range of conditions for the conversion of oxygenates, such as methanol, to olefins as well as for the oligomerization of olefins to higher molecular weight hydrocarbons. Therefore, it is not altogether unexpected that, even under the relatively mild conditions used for the production of alkyl tert-alkyl ether in contact with zeolite catalyst, some olefin oligomerization would take place with the consequent production of undesirable by-product. In order to fully realize the potential of zeolite Beta catalysis in the production of high octane tertiary ethers, particularly MTBE, there is a need to provide a means to reduce the formation of olefin oligomer by-product.
It is an objective of the present invention to provide a process for the improved production of alkyl tertiary alkyl ether using zeolite catalyst.
A specific objective of the present invention is to provide a process for the improved production of alkyl tertiary alkyl ether using zeolite Beta catalyst.
Another objective of the present invention is to provide a process for the improved production of alkyl tertiary alkyl ether using zeolite Beta catalyst while reducing the formation of olefin oligomer by-product.